This invention relates to a ball assembly for use in activating a by-pass tool in a drill string.
During drilling through the earth's crust in order to reach underground reservoirs of hydrocarbons (gas and/or oil), it is usual to employ a so-called “drill string, and which is driven from the surface, and has a drilling bit on its lower end. It is also usual to employ drilling mud which is conveyed from the surface to the drilling bit via the drill string, in order to lubricate and cool the bit, but which then returns to the surface via the annulus between the drill string and the usual surrounding casing, and also conveying to surface at the same time the “cuttings” formed during the drilling operation.
During typical drilling operations, problems often arise because of differences in the pressures in the geological formation being drilled and at the surface, or between the pressure of the drilling mud and the formation pressure. Major problems arising out of this include blow-outs, differential sticking and mud circulation loss. Any of these problems can be very dangerous, and often require expensive solutions.
A preferred solution to the problem is to provide a by-pass tool in the drill string, and which includes a through-flow housing through which the mud can flow, and then onwards to the drilling bit, when the tool is operating in a normal de-activated mode. However, when a problem arises (e.g. a lost circulation condition, when drilling fluid is being lost to the formation, and it is desired to inject lost circulation material into the formation), the tool is then activated so that the drilling mud is diverted laterally through a by-pass port in the wall of the housing, and no longer flows downwardly through the housing.
In order to activate the by-pass tool, it is known to use an activating ball which is launched down the drill string from the surface, and which moves down the drill string until it comes into engagement with a valve seat in the tool. This then activates the tool, so that drilling mud in the drill string above the tool can no longer flow downwardly through the tool, but is diverted laterally through the by-pass port.
One example of such an arrangement is disclosed in U.S. Pat. No. 4,889,199, and in which the activating ball is a large deformable ball made of plastics material, and which engages the valve seat (which is provided in a linearly displaceable control sleeve forming part of the tool), and as the mud pressure above the ball builds-up, the ball urges the sleeve downwardly against spring biassing, and so as to allow access for the mud to the by-pass port.
When it is required to de-activate the tool, a second small (and hard) ball is launched down the drill string, and which comes to rest above the larger deformable ball and at the same time blocks access to the by-pass port This stops the transverse by-pass flow of mud, and therefore the pressure above the ball again increases, and when it reaches a certain level, the larger ball is deformed inwardly so that both balls can now pass downwardly through the tool (usually to be received by a lower ball catcher device). The control sleeve then returns under its spring biassing to its original position, so that through-flow of mud lengthwise of the housing can resume.
The use of an activating ball (the large deformable ball) and the de-activating ball (the small hard ball) works very well in practice, and is a very useful feature available to drilling operators. However, while the large deformable ball is well able to move downwardly of the drill string to engage the valve seat when there is pumped mud pressure available in the drill string above the ball, it is much slower in its movement when pumped pressure is not available. In such a situation, the ball can then move downwardly under gravity action only, and therefore moves more slowly before it comes into engagement with the valve seat.